项目描述
Windows Phone 7的文本框控件，给用户一个指引，文本框需要填写的内容。
就像内置的日历中填写预约时相同。该控件继承了TextBox类。
A Windows Phone 7 TextBox control, providing Watermark text to add a visual clue to the user for what he has to enter in the field, like the built-in Calendar app when you want to create a new appointment.
The control inherits TextBox.
